The United States of America (USA), commonly referred to as the United States (U.S.) or America, is a federal republic composed of 50 states, a federal district, five major self-governing territories, and various possessions.[fn 1] The 48 contiguous states and federal district are in central North America between Canada and Mexico, with the state of Alaska in the northwestern part of North America and the state of Hawaii
comprising an archipelago in the mid-Pacific. The territories are scattered about the Pacific Ocean and the Caribbean Sea. At 3.8 million square miles (9.8 million km2) and with over 324 million people, the United States is the world's third largest country by total area (and fourth largest by land area)[fn 2] and the third most populous. It is one of the world's most ethnically
diverse and multicultural nations, the product of large-scale immigration from many other countries. The geography and climate are also extremely diverse, and the country is home to a wide variety of wildlife.
Paleo-Indians migrated from Asia to the North American mainland at least 15,000 years ago, with European colonization beginning in the 16th century. The United States emerged from 13 British colonies along the East Coast. Numerous disputes between Great Britain and the colonies in the aftermath of the Seven Years War led to the American Revolution, which began in 1775. On July 4, 1776, as the colonies were fighting Great Britain in the American Revolutionary War, delegates from the 13 colonies unanimously adopted the Declaration of Independence. The war ended in 1783 with recognition of the independence of the United States by Great Britain, and was the first successful war of independence against a European colonial empire. The current constitution was adopted in 1788, after the Articles of Confederation, adopted in 1781, were felt to have provided inadequate federal powers. The first ten amendments, collectively named the Bill of Rights, were ratified in 1791 and designed to guarantee many fundamental civil liberties.
The United States embarked on a vigorous expansion across North America throughout the 19th century, displacing American Indian tribes, acquiring new territories, and gradually admitting new states until it spanned the continent by 1848. During the second half of the 19th century, the American Civil War led to the end of legal slavery in the country. By the end of that century, the United States extended into the Pacific Ocean, and its economy, driven in large part by the Industrial Revolution, began to soar. The Spanish–American War and World War I confirmed the country's status as a global military power. The United States emerged from World War II as a global superpower, the first country to develop nuclear weapons, the only country to use them in warfare, and a permanent member of the United Nations Security Council. The end of theCold War and the dissolution of the Soviet Union in 1991 left the United States as the world's sole superpower.
The United States is a highly developed country, with the world's largest economy by nominal GDP. It ranks highly in several measures of socioeconomic performance, including average wage, human development, per capita GDP, and productivity per person. While the U.S. economy is considered post-industrial, characterized by the dominance of services, the manufacturing sector remains the second largest in the world. Though its population is only 4.4% of the world total, the United States accounts for nearly a quarter of world GDP and almost a third of global military spending, making it the world's foremost military and economic power. The United States is a prominent political and cultural force internationally, and a leader in scientific research and technological innovations.